Method and device for purification of a liquid

ABSTRACT

Method for purification of a liquid, which method comprises the steps of; conducting the liquid through a purifying chamber ( 19 ), activating a UV light source ( 22 ) comprising a gas for lighting and illuminating the liquid in the chamber by the UV-light source. In order to increase the efficiency and shortening the start-up time of the UV light source, the gas is heated to an increased temperature prior to activating the UV light source for lighting, using a heating element which is placed outside the WV light source.

FIELD OF THE INVENTION

The present invention relates to a method for purifying liquidcomprising: passing liquid through a purification chamber and activatinga UV light source for lighting up, which UV light source contains a gasand is arranged in the purification chamber, illuminating the liquid inthe tube with UV light by means of the UV light source when this is litup. The invention also relates to a liquid purifier for carrying out themethod. The method and the liquid purifier are particularly suitable tobe used for water purification in caravans, campers, households andother similar applications.

BACKGROUND ART

In for example campers, there is a need to carry and store water fordomestic requirements. The water is normally stored in a storage tank,which contains the normal requirements for anything from a couple ofdays up to several weeks. However, it is common for at least some of thequantity of water to remain in the tank for a considerably longer periodof time, for example if the tank is not emptied completely and cleanedbefore each refill. Such long-term storage can give rise to the growthof micro-organisms which can be harmful to the user, particularly if thewater is used as drinking water. Tests have shown that water which isstored for a week in the storage tank of a camper exceeds the guidelinesof the Swedish. National Food Administration relating to the presence ofmicro-organisms in drinking water by a factor of 28. Therefore the watersupply system may be equipped with a water purifier, which is connectedin series between the storage tank and the tapping points. In such acase, the water supply system can comprise a storage tank, a pump, awater purifier, a water heater and one or more tapping points in theform, for example, of faucets. One type of water purifier comprises afilter which is connected via a reducing valve to a UV-light purifier,which comprises a purification chamber with a UV light source and awater pipe, for example a quartz glass tube, which allows UV light topass through.

When a user wants to draw off water, he or she must first ensure thatthe UV light source is shining with sufficient intensity. Thereafter theuser can start the pump. This is carried out either by the user openingthe faucet at a tapping point, whereupon a pressure sensor in the systemor an automatic switch in the faucet starts the pump, or, alternatively,the pump can be started manually by a switch at the tapping point. Whenthe pump starts, water is pumped from the tank to the water purifier,where it first passes through the filter with active carbon to filteroff solid impurities, chlorine, smell and taste. From the filter, thewater is taken via the reducing valve to the purification chamber. Inthe purification chamber, the water is passed through a quartz glasstube which is arranged parallel to a UV fluorescent tube. A reflector isnormally arranged parallel to the longitudinal axis of the quartz glasstube and the UV fluorescent tube, surrounding these elements, so thatthe UV light is focused towards the central longitudinal axis of thequartz glass tube, as described, for example, in WO 96/33135. When thewater passes through the quartz glass tube, it is illuminated by the UVlight, whereby the micro-organisms that have passed through the filterare exposed to UV radiation. This affects certain molecular structuresin the micro-organisms, whereupon these die or at least are madeharmless for a period of time. When the water has passed through thequartz glass tube, it is taken out of the purification chamber and, ifrequired via a heater, to the tapping point.

In order to ensure that sufficiently many or all the micro-organisms aredestroyed or made harmless, it is important that the dose of UVradiation to which they are exposed is sufficiently high. The dose ofradiation depends upon the intensity of the UV radiation and the flow ofliquid through the quartz glass tube, so that the dose increases withhigher intensity and reduces with higher flow. As the user-criteria laydown requirements for a particular minimum flow, it is important thatthe UV intensity is kept sufficiently high in order to ensure that thereis sufficient destruction of micro-organisms.

The most common type of UV light source in the water purifiers describedabove consists of a so-called UV fluorescent tube. These comprise anelongated cylindrical glass tube containing a gas, normally rarefiedargon gas with mercury vapour. At each end of the tube there is anelectrode. Further, the electrodes are connected to a time-controlledrelay or a glow switch and to a voltage source. When the UV fluorescenttube is activated for lighting up, the voltage source is connected andthe relay closes, so that a current goes from the voltage source to oneelectrode, through the closed relay and through the second electrodeback to the voltage source. As the electrodes have a certainresistivity, they heat up during this short activation phase forlighting up the UV fluorescent tube. After a short time, normally around0.1-1.0 seconds, the relay drops out. One electrode, the cathode, hasthen started to glow. The voltage is still passing across the electrodesand the cathode will hereby emit electrons that move freely through theglass tube to the second electrode, the anode. When the electrons movefreely through the tube, they collide with gas molecules, whereuponradiation in the form of UV light starts to be emitted, that is to saythe fluorescent tube lights up.

A problem with the use of such UV fluorescent tubes in water purifiersis that the fluorescent tube does not attain its full radiationintensity until the gas has reached a certain temperature, normallyaround 30-40° C. In particular in mobile applications for waterpurifiers, such as in caravans and campers, this is a problem, as thesurrounding temperature can be very low. It is true that the gas and theelectrodes are heated up by the heat that is generated when thefluorescent tube is shining, but if the surrounding temperature is, forexample, 5° C. when the UV fluorescent tube is activated for lightingup, it can take several minutes before the fluorescent tube has reachedeven 80% of its full intensity. The problem hereby arises that theadequate destruction of micro-organisms cannot be ensured during thetime that it takes for the gas to reach a temperature of around 30° C.

One attempt to solve this problem is to greatly over-dimension the UVfluorescent tube, so that, even with low surrounding temperatures, theinitial intensity is sufficiently high to ensure adequate destruction ofmicro-organisms. This solution is, of course, not satisfactory, as whenthe UV fluorescent tube has reached optimal temperature it would then beoperating with several hundred percent over-capacity and as both theprice of the purifier and the running costs would increase considerably.Another attempt to solve the problem is to introduce a time-delay intothe water supply system, so that the pump cannot be started until acertain time after the UV fluorescent tube has been activated. In orderto ensure adequate destruction, even in use in cold climates, the timedelay must be set to be several minutes, which has proved to beunacceptable to users or at least very annoying. Yet another attempt tofind a solution is to let the UV fluorescent tube shine continuouslywith full intensity, even when the water supply system is not beingused, in order to attempt to maintain a raised temperature. Thissolution is, however, impractical, as the energy consumption would bevery high and the life of the UV fluorescent tube would be greatlyreduced, resulting in frequent, expensive and complicated replacementsof the fluorescent tube.

U.S. Pat. No. 5,738,780 describes a water-treatment system in which a UVlamp is operated at full intensity when the water flows through thesystem and at a lower intensity when the system is not being used. Bythis means, it is said that the time to attain full intensity of thelamp is reduced. The system described in U.S. Pat. No. 5,738,780 has,however, certain disadvantages. The lamp must be operated continuously,which means that an electrical current continuously passes through thelamp's filaments. This means that wear on the lamp increases and thatits life is thereby reduced. In addition, the system described comprisesno means for controlling the temperature in the lamp or in thepurification chamber around the lamp. As described above, the time toattain full light intensity depends upon the temperature of the lamp.The system described in U.S. Pat. No. 5,738,780 makes it possible forthe lamp to attain full intensity relatively quickly when it isactivated, provided that the surrounding temperature is sufficientlyhigh. With low surrounding temperatures, however, the lamp will still berelatively cold in the low-intensity mode, for which reason the time toattain full intensity is prolonged.

An alternative UV source that can be used in water purifiers of the typedescribed above is an induction lamp. Such lamps comprise a double-shellglass bulb with a metal in gaseous form between the shells. Inside theglass bulb is an inductor in the form of an electrical coil. When theinductor is supplied with high-frequency AC voltage, a magnetic field isinduced, which causes the metal gas to emit UV light. The problemsdescribed above, which are associated with attaining optimal operatingtemperature, can also occur with the use of induction lamps.

BRIEF DISCLOSURE OF INVENTION

An object of the present invention is therefore to achieve a method forpurifying liquid according to the first paragraph of this description,which method in a simple and cost-effective way eliminates or greatlyreduces the problems that arise as a result of the UV light source notattaining full intensity until the gas in the UV light source hasreached a certain temperature.

This object is achieved by heating up the gas to a raised temperaturerelative to the surroundings outside the purification chamber by meansof a heat-generating element which is arranged outside the UV lightsource, in a standby mode prior to the activation for lighting up. Bymeans of this, it is possible, for example, to keep the gas in the UVlight source continuously at approximately the optimal temperature,without the UV light: source being lit up. The UV light source will thenemit full intensity almost immediately after it has been lit up. In thisway, it is ensured that there is sufficient destruction or renderingharmless of the micro-organisms in the first quantity of water that isdrawn off from the water supply system, even if the pump is startedimmediately after or at the same time as the UV light source isactivated for lighting up. At the same time, the life of the UV lightsource is not reduced, as it only needs to be lit up when the pump isworking and water is being drawn off from the system.

According to an embodiment of the method, the gas in the UV light sourceis heated up using an electrical resistance, which is arranged in thepurification chamber in such a way that heat from the resistance istransmitted to the gas in the UV fluorescent tube by radiation andconvection through the air that surrounds the UV light source in thepurification chamber. By this means, a simple solution is achieved,which is relatively cheap both to manufacture and to run.

According to a preferred embodiment of the method, the temperature inthe purification chamber is measured continuously, while at the sametime the heating up of the gas in the UV light source is regulateddepending upon the measured temperature. By this means, it is ensuredthat the temperature of the gas in the UV light source can be keptwithin the optimal range irrespective of variations in the surroundingtemperature. In addition, the temperature control results in improvedrunning economy, as the heating is switched off or reduced if thesurrounding temperature increases.

Another object of the invention is to achieve a liquid purifier forcarrying out the method. The liquid purifier comprises a purificationchamber, in which a tube through which water passes and a UV lightsource are arranged in such a way that the UV light source, when it islit up, illuminates the water in the tube with UV light. The liquidpurifier according to the invention is characterized by means forcontrolled heating up of the gas in the UV light source, which meanscomprises a heat-generating element which is arranged outside the UVlight source. By controlled heating up of the gas in the UV light sourceis meant here that the temperature of the gas can be kept above apredetermined temperature irrespective of the temperature of thesurroundings outside the purification chamber and irrespective of theheat generation that can occur in the UV light source when this isactivated for lighting up and when it is shining.

According to an embodiment of the liquid purifier according to theinvention, the means for controlled heating up comprises an electricalresistance which is arranged in the purification chamber in such a waythat when it is supplied with an electrical current, it generates heatwhich is transmitted to the gas in the UV light source by radiation andconvection in the air-filled purification chamber.

The liquid purifier may also comprise a temperature sensor which isarranged in the purification chamber in order to measure the temperaturein the purification chamber. The sensor is connected to a control devicewhich controls the resistance so that the temperature is kept within apredetermined range, irrespective of variations in the surroundingtemperature and irrespective of any heat generated by the UV lightsource when this is shining.

In order to minimize the energy consumption, heat insulation may bearranged around the purification chamber. The heat insulation issuitably constructed of a material that has good heat-insulatingproperties and that is resistant to UV radiation. An example of such amaterial is expanded propene plastic, EPP.

DESCRIPTION OF DRAWINGS

An exemplified embodiment of the method and the liquid purifieraccording to the invention is described below, with reference to theattached figures in which:

FIG. 1 shows schematically a water supply system which can be used, forexample, in campers.

FIG. 2 is a section through a UV-light purifier in a liquid purifier forcarrying out the method according to the invention.

FIG. 3 is a diagram showing how the UV light intensity varies over timein a liquid purifier according to the invention and in a liquid purifieraccording to current technology.

FIG. 1 shows schematically a water supply system, for example forcaravans, campers, boats, planes or any similar application, where thewater for domestic requirements is carried in a storage tank 1. Thewater supply system comprises a pump 2, the suction side of which isconnected to the storage tank 1 by a pipe 3. The pressure side of thepump 2 is connected via a pipe 4 to an inlet 5 of a water purifier 6.The water purifier 6 comprises a filter 7 which contains active carbon,a reducing valve 8 and a UV-light purifier 9 which will be described ingreater detail below. An outlet 10 of the water purifier is connected toa cold water faucet 12 at a tapping point 13 via a pipe 11. The outlet10 is also connected via a pipe 14 to a water heater 15 powered byelectricity or bottled gas, which is connected to the hot water faucet17 at the tapping point via a pipe 16. A pressure sensor 2 a isconnected to the pressure side of the pump in order to detect the liquidpressure in the system.

Alternatively, the pressure sensor 2 a can be arranged in any othersection of pipe between the pump 2 and the tapping point 13.

FIG. 2 shows the UV-light purifier 9 which is incorporated in the waterpurifier 6. The UV purifier 9 comprises a casing 18 which is arrangedaround an elongated purification chamber 19 which is laterally delimitedby a reflector 20. Heat insulation 21, for example of expanded propeneplastic (EPP) or some other heat-insulating material, is arrangedbetween the reflector 20 and the casing 18. A UV fluorescent tube 22 isarranged in the purification chamber 19, parallel to its longitudinalaxis. The fluorescent tube 22 comprises a glass tube 23 which is filledwith, for example, rarefied argon gas and mercury vapour. At one end 23a of the glass tube 23 there is an electrode 27 in the form of an anode27 a and at the other end 23 b there is an electrode in the form of acathode 27 b. The anode 27 a and the cathode 27 b are connected to atime-controlled relay or a glow switch and a voltage source (not shown).A water pipe 24 is arranged in the purification chamber 19, parallel toits longitudinal axis and to the fluorescent tube 22. The water pipe 24is made of a material that is water-tight but that passes through UVlight. In the example shown, the water pipe consists of a quartz glasstube 24, but a thin-walled tube of teflon™ or other material may also beused. The quartz glass tube 24 has in addition an inlet 24 a, which isconnected to the filter 7 via a pipe and a reducing valve 8, and anoutlet 24 b which is connected to the tapping point 13 of the watersupply system (see FIG. 1). The reflector 20, the fluorescent tube 22and the quartz glass tube 24 are arranged in such a way that the UVlight from the fluorescent tube is reflected by the reflector 20 and isfocused along the central longitudinal axis of the quartz glass tube. Tomake this possible, the reflector 20 and hence the purification chamber19 have a generally elliptical cross section. The design of thereflector 20 and the position of the fluorescent tube 22 and the quartzglass tube 24 in the reflector are described in greater detail in WO96/33135.

Immediately outside the purification chamber 19, essentially on a levelwith its centre, there is a circuit board 25. The circuit board carriescomponents for monitoring and controlling the function of the UVpurifier. These components are described in greater detail in theSwedish patent application entitled “System for supplying liquid” withthe same applicant and filing date as the present patent application.The components comprise a UV-light sensor 26 for measuring the intensityof the UV fluorescent tube. The UV-light sensor 26 is attached to thecircuit board 25 and arranged in such a way that it projects slightlyinto the purification chamber, through an opening in the reflector 20.During operation, the UV-light sensor 26 measures the UV intensity inthe purification chamber 19. If the intensity drops, for example as aresult of the UV fluorescent tube 22 being worn out, the output signalfrom the sensor 26 drops to a low level. This is detected by amicroprocessor (not shown) on the circuit board, which stops the pump 2and lights up a warning lamp or diode (not shown). The user then knowsthat the UV purifier is not working normally and that it needsattention. Instead of a UV-light sensor 26, a sensor for detectingvisible light may be used. In such a case, the microprocessor carriesout a conversion in order to calculate the UV light intensity on thebasis of the measured intensity of the visible light.

Such sensors for detecting UV or visible light usually degenerate ifthey are subjected to long-term exposure to UV light. In order to shieldthe UV-light sensor 26 from UV radiation when the UV intensity does notneed to be measured, a sensor shield 28 is arranged inside the quartzglass tube 24. The sensor shield 28 can move axially in the quartz glasstube between a lower position A where it shades the light sensor 26 fromUV radiation and an upper position B where the UV light can freelyradiate from the UV fluorescent tube 22 to the light sensor 26. When thepump 2 (FIG. 1) is in operation, water is pumped through the quartzglass tube 24, whereupon the sensor shield 28 moves with the flow ofwater to the position B. The light sensor 26 can then measure the UVlight intensity. When the pump 2 stops, the flow in the quartz glasstube 24 stops, whereupon the sensor shield 28 sinks down to the positionA and shields the light sensor 26 from UV radiation when measurement ofthe intensity does not need to be carried out. The sensor shield 28 canalso be used for indicating that the filter 7 is blocked. If the filter7 is blocked, the flow through the quartz glass tube 24 is reduced,whereupon the sensor shield 28 sinks down to position A and blocks theUV radiation to the light sensor 26. The output signal from the lightsensor 26 then drops to a low level, which is detected by themicroprocessor, which stops the pump and lights up the warning lamp ordiode. A flow shield 29 can be arranged to move axially in the quartzglass tube 24 between a lower position C and the position A. The flowshield 29 is designed to be in the lower position C in the event ofnormal flow through the quartz glass tube 24. If the flow through thequartz glass tube 24 rises above the normal level for any reason, sothat adequate destruction of micro-organisms cannot be guaranteed, theflow shield rises to position A where it shades the light sensor 26. Theoutput signal from the light sensor 26 then drops to a low level, whichis again detected by the microprocessor, which stops the pump and lightsup the warning lamp or diode.

According to one embodiment of the liquid purifier according to theinvention, the liquid purifier comprises a resistance 30 and atemperature sensor 31. The resistance 30 is attached to the circuitboard 35 and arranged in such a way that it projects slightly into thepurification chamber, through an opening that is made in the reflector20. The resistance is connected to a voltage source (not shown) and isnormally supplied with 12 or 24 V DC. With full voltage supply, theresistance generates around 1-10 W, preferably around 24 W heatingeffect. The task of the resistance is to heat up the gas in the UVfluorescent tube and to keep the temperature of the gas within the rangeat which the UV fluorescent tube produces the optimal radiationintensity. In certain applications, the optimal radiation intensity canbe 100% of the maximum intensity, but it is more usual for the optimalradiation intensity to be around 80% of the maximum intensity of the UVlight source. The temperature range of the gas within which the UV lightintensity is around 80% of the maximum normally starts in excess ofaround 25° C. and is usually in particular between around 30° C. and 40°C. The heat from the resistance 30 is transmitted to the gas in the UVfluorescent tube 22 by radiation and convection through the air thatsurrounds the UV fluorescent tube in the purification chamber 19. As theresistance projects into the purification chamber, an effective heattransmission to the air and the UV fluorescent tube in the purificationchamber is obtained when the resistance is in operation.

The temperature sensor 31 is also attached to the circuit board 25 andarranged in such a way that it projects slightly into the purificationchamber 19, through an opening that is made in the reflector 20. Thetemperature sensor 31 and the resistance 30 are connected electricallyvia a regulating device (not shown) in such a way that the heatingeffect that is generated by the resistance 30 is controlled dependingupon the air temperature in the purification chamber 19 which ismeasured by the temperature sensor 31. This can either be carried out byintermittent operation of the resistance 30, so that the resistance issupplied with a constant voltage as long as the temperature in thepurification chamber is below a certain threshold value, for example 30°C., and the resistance 30 is not supplied when the temperature in thepurification chamber 19 is above this value. Alternatively, the supplyvoltage for the resistance 30 can be varied in relation to the measuredtemperature in the purification chamber 19, so that the supply voltageand thereby the generated heating effect is reduced when the temperaturerises and approaches for example 35° C. and in a corresponding way isincreased if the temperature drops below for example 30° C. After acertain period of heating up of the air in the purification chamber 19and the gas in the UV fluorescent tube 22, the gas and the air will beapproximately the same temperature. With continuous temperaturemonitoring and resultant controlled heating, the air temperature that ismeasured by the temperature sensor 31 will therefore also be valid forthe temperature of the gas in the UV fluorescent tube 22.

For the use of the water supply system in, for example, a camper, theresistance 30 can be continuously connected to the voltage source viathe regulating device, so that the temperature is kept constantly abovefor example 25° C. and suitably between 30° C. and 40° C., irrespectiveof the temperature of the surrounding atmosphere outside thepurification chamber. The heating system is, however, suitably connectedto the main switch of the vehicle, so that the heating is disconnected,for example when the camper is not used for a shorter or longer periodof time.

With reference to FIGS. 1 and 2, it is described below how theembodiment of the invention described above is used. When a user wantsto draw off water from the water supply system, he or she opens one orboth faucets 12, 17 at the tapping point 13. The pressure sensor 2 a inthe system detects that the pressure drops, whereupon the pump isstarted and the UV fluorescent tube 22 is activated for lighting up.This activation for lighting up is carried out by means of either atime-controlled relay or a glow switch, in the normal way forfluorescent tubes. A voltage is applied to the UV fluorescent tube 22,across the relay or glow switch, the anode 27 a and the cathode 27 b.The relay or glow switch then closes, so that a current passes from theanode, via the relay or glow switch to the cathode. After a moment, thecathode 27 b has started to glow and the relay or glow switch drops out.Electrons are then emitted from the cathode, whereupon the fluorescenttube 22 lights up. This activation phase for lighting up of thefluorescent tube lasts in the order of 0.1-1 second if a time-controlledrelay is used and 1 to 3 seconds if a glow switch is used.

As the tecmperature of the gas in the UV fluorescent tube 22 has alreadybeen reached and maintained within the predetermined temperature rangedescribed above before the activation for lighting up of the fluorescenttube, the UV fluorescent tube will emit the optimal radiation intensityalmost immediately after it has been lit up. By this means, it isensured that the micro-organisms that are to be found in the quartzglass tube 24 right from the start are exposed to a sufficiently highdose of UV light to be destroyed or rendered harmless.

FIG. 3 shows the result from a test that was carried out using a UVliquid purifier according to current technology and one according to theembodiment of the invention described above. Both liquid purifiers wereidentical with the exception of the liquid purifier according to theinvention being provided with the system, described above for heatingthe gas in the UV fluorescent tube. The diagram shows the intensity (I.)of the UV fluorescent tube as a percentage of the maximum intensity as afunction of the time (t.) in seconds from the UV fluorescent tube beingactivated for lighting up. During the tests, the temperature of thesurrounding atmosphere outside the purification chamber was 5° C. Thelower curve shows the relationship for the conventional UV liquidpurifier and the upper curve shows the relationship for the UV liquidpurifier according to the invention.

The diagram shows that it took approximately 460 seconds before the UVlight intensity in the conventional purifier reached 80% of the maximumintensity, while the corresponding time for the purifier according tothe invention was less than 20 seconds. By means of the method and theliquid purifier according to the invention, it is thus significantlyquicker to attain the UV light intensities that are required in order toensure adequate destruction or rendering harmless of micro-organisms.

According to an alternative embodiment (not shown) of the liquidpurifier according to the invention, the means for heating the gas inthe UV fluorescent tube consists of a resistive heat-generatingelectrical cable which is wound in the form of a spiral around the glasstube of the UV fluorescent tube. This resistive cable can be connectedto a temperature sensor and regulating device in a corresponding way tothe resistance above and it also works in a corresponding way, with theexception that the heat generation takes place closer to the gas.

According to another embodiment (not shown), the means for heating thegas consists of the UV fluorescent tube's electrodes. In this case, theresistivity of the fluorescent tube's electrodes is used to heat the gasin the fluorescent tube. For this purpose, an electrical shorting of thefluorescent tube's time-controlled relay or glow switch is arranged sothat a heating current can be passed through the electrodes for a longerperiod of time without the relay or glow switch dropping out and thefluorescent tube lighting up. During the activation phase for lightingup and during normal operation of the UV fluorescent tube after it haslit up, the UV fluorescent tube is normally operated with 50 V AC. Thevoltage that is used to heat the electrodes in the fluorescent tubeaccording to the embodiment described here should, however, beapproximately 12 V DC. For this reason, means must be arranged forsupplying the fluorescent tube with 12 V DC during the heating up phaseand with 50 V AC during the activation phase for lighting up and duringnormal operation. These means can, for example, consist of two differentvoltage sources with a change-over switch or with a transformer with twodifferent supply outputs. When the heating current is passed through theUV fluorescent tube's electrodes, these generate heat inside the UVfluorescent tube, so that the gas in the glass tube is heated up. Thisway of heating up the gas can also be combined with a temperature sensorin the purification chamber, which indirectly measures the temperatureof the gas and with a regulating device for controlling the heatingdepending upon the measured temperature.

The method and the liquid purifier according to the invention can alsobe used with corresponding effects and advantages in a liquid purifierwhere the UV light source consists of an induction lamp.

It is described above how the method and the liquid purifier are used ina camper or other vehicle. The invention can, however, also be used forother applications, such as stationary households, where the storagetank can be replaced by, for example, a well or a connection to apressurized water supply network. In the latter case, the pump can bereplaced by an electrically-controlled valve.

The time that the liquid purifier is in its standby mode, that is whenthe heating system is in operation, can vary between differentapplications. A continuous standby mode is described above. It is,however, also possible for the liquid purifier to assume its standbymode for shorter periods that can be controlled by, for example, theuser or a timer. In such a case, it is important that the standby modeis assumed a sufficiently long time before the water is to be drawn offfrom the system, in order that the gas in the UV fluorescent tube can beheated to the predetermined temperature.

1. Method for purifying liquid comprising: passing liquid through apurification chamber (19), activating a UV light source (22) forlighting up, which UV light source contains a gas and is arranged in thepurification chamber, illuminating the liquid in the purificationchamber with UV light by means of the UV light source, when this is litup, characterized by heating up the gas to a raised temperature inrelation to the surroundings outside the purification chamber, in astandby mode prior to activation for lighting up, by means of aheat-generating element (30) which is arranged outside the UV lightsource.
 2. Method according to claim 1, wherein, in the standby mode,the gas is heated up by passing an electrical current through aresistive heat-generating element (30).
 3. Method according to claim 1or 2, comprising measuring the temperature in the purification chamber(19) and controlling the heating up in relation to the measuredtemperature.
 4. Method according to claim 1, wherein, in the standbymode, the gas is heated up to a temperature above 25° C., preferablybetween 30° C. and 40° C., and thereafter is maintained at essentiallythis temperature in the continued standby mode and after the UV lightsource (22) has been lit up.
 5. Liquid purifier comprising apurification chamber (19), in which a tube (24) through which waterpasses and a UV light source (22) which contains a gas are arranged insuch a way that the UV light source, when it is shining, illuminates theliquid in the tube with UV light, characterized by means for controlledheating up of the gas in the UV light source, which means comprises aheat-generating element (30) which is arranged outside the UV lightsource.
 6. Liquid purifier according to claim 5, in which the means forcontrolled heating up of the gas comprises a resistive heat-generatingelement (30), which is arranged in the purification chamber (19) outsidethe UV light source (22) for heating up the gas in the UV light sourceby radiation and convection in the purification chamber (19).
 7. Liquidpurifier according to claim 5 or 6, in which the UV light sourcecomprises a fluorescent tube, characterized in that the means forheating up the gas comprises a resistive electrical cable that isarranged around at least part of the outside of the fluorescent tube. 8.Liquid purifier according to claim 5, comprising a device (31) formeasuring the temperature in the purification chamber (19), which deviceis connected to a regulating device for controlling the controlledheating up in relation to the measured temperature.
 9. Liquid purifieraccording to claim 5, in which the purification chamber (19) is heatinsulated.